It isn't just about the brightness (range). In practice the 'HDR' standards are also about wider color gamuts (than sRGB), and (as mentioned in parallel) packe

It isn't just about the brightness (range). In practice the 'HDR' standards are also about wider color gamuts (than sRGB), and (as mentioned in parallel) packed into more bits, in a different way, so as to minimise banding while keeping file sizes in check.